. Cunningham's Text-book of anatomy. Anatomy. THE TEACHEA. 1079 These differences in the calibre of the tube are determined by the surroundings of the trachea. The cervical part is narrowed owing to its being clasped by the thyreoid gland. Further, a short distance above its bifurcation an impression, sometimes strongly marked, is usually seen on the left side of the trachea. This is due to the close contact of the aortic arch as it passes dorsally against this part of the tube. It is evident therefore that the second slight diminution in calibre which is described by Braune and Stahel is pro-
. Cunningham's Text-book of anatomy. Anatomy. THE TEACHEA. 1079 These differences in the calibre of the tube are determined by the surroundings of the trachea. The cervical part is narrowed owing to its being clasped by the thyreoid gland. Further, a short distance above its bifurcation an impression, sometimes strongly marked, is usually seen on the left side of the trachea. This is due to the close contact of the aortic arch as it passes dorsally against this part of the tube. It is evident therefore that the second slight diminution in calibre which is described by Braune and Stahel is pro- duced by the proximity of the aorta. Lejars gives the average ventro-dorsal diameter of the trachea in the living person as 11 mm., and the transverse diameter as 12-5 mm. In the dead subject the lumen of the tube is considerably Thyreoid cartilage -Crico-thyreoid membrane — Cricoid cartilage _Part of trachea covered by isthmus of thyreoid body Common carotid artery Subclavian artery The trachea adheres rigorously to the median plane except towards its termination, where it de- viates very slightly to the right. As it passes down- wards it recedes rapidly from the surface. This is due to its following the curvature of the vertebral column, from which it is separated by the oesophagus alone. Relations of the Trachea.—In the study of the relations of the trachea it is convenient to consider it in the two stages of cer- vical and thoracic. When the chin is held so that the face looks directly forwards the cervical part of the trachea measures from 2 to 2\ inches in length ; but when the head is thrown dorsally or backwards the length is considerably in- creased. It is clasped by the thyreoid gland, the isthmus of which is applied to its ventral surface, and covers the second, third, and fourth rings; while on each side the lateral lobe of the thy- reoid gland is applied to the lateral surface of the trachea and extends downwards to the fifth or sixth ring. On either
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